Voxel data in Cycles
You can use a particle grid to get back the ability to use a Voxel data texture.


Create a box with the proportions of the voxel data
add a particle system
set the emission to Volume and Grid
set the end emission frame to 1
set a low resolution at first (like 40)
add a texture to the particle system
set the texture to Voxel data
load your voxel data file
Enable Ramp to tweak the threshold
Set the influence to Density




If your volume doesn't appear, it's just because of a little bug that you can easily get around by switching to Blender Internal Render View mode and back to Cycles Solid mode, then refreshing the Voxel Data. The bug is reported here.


add a material
Replace the Diffuse by a Volume Scatter and plug it to Volume
Add a Point Density node and plug it to the density of the Volume scatter
Plug a multiply node inbetween to get a higher density
In the Point Density node, select the other box object and its particle system





tweak the resolution settings of both the particle grid and the point density. Note that the particle grid resolution is limited to 250. If you want a higher resolution you have to slice your voxel data in pieces of 250x250x250 (or any resolution below) and use multiple particle grids. I won't do that because my computer is a toaster but I think you get the idea.


You can then add a particle killer mesh to cut your MRI where you want :



This is ugly because the resolution is low but it can be as good as your computer allows it to be.

Here's a file you can open to test (provided you also put your MRI file in the same folder as the .blend) :